riscv, bpf: Enable inline bpf_kptr_xchg() for RV64

RV64 JIT supports 64-bit BPF_XCHG atomic instructions. At the same time,
the underlying implementation of xchg() and atomic64_xchg() in RV64 both
are raw_xchg() that supported 64-bit. Therefore inline bpf_kptr_xchg()
will have equivalent semantics. Let's inline it for better performance.

Signed-off-by: Pu Lehui <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Acked-by: Björn Töpel <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

arch/riscv/net/bpf_jit_comp64.c

···
 {
 	return true;
 }
+
+bool bpf_jit_supports_ptr_xchg(void)
+{
+	return true;
+}